A fuel cell, which is a cell that directly converts chemical energy generated by oxidation of a fuel into electrical energy, attracts much attention as a next-generation energy source owing to high energy efficiency and high eco-friendliness based on less contaminant exhaust.
In general, the fuel cell has a structure in which an oxidation electrode (anode) and a reduction electrode (cathode) are opposite to each other via an electrolyte membrane and such a structure is referred to as a “membrane electrode assembly (MEA)”.
Depending on the type of electrolyte membrane, the fuel cell is classified into an alkaline electrolyte fuel cell, a direct oxidation fuel cell, a polymer electrolyte membrane fuel cell (PEMFC) and the like. Among them, the polymer electrolyte fuel cell comes into the spotlight for portable, vehicle and household power generation applications due to advantages such as low operation temperature of less than 100° C., rapid starting and response speeds and excellent durability.
Representative examples of the polymer electrolyte fuel cell include a proton exchange membrane fuel cell (PEMFC) using a hydrogen gas as a fuel, and the like.
An overall reaction occurring in the polymer electrolyte fuel cell will be described in brief. First, when a fuel such as hydrogen gas is fed to the anode, hydrogen is oxidized at the anode to produce a hydrogen ion (H+) and an electron (e−). The produced hydrogen ion (H+) is transferred to the cathode via the polymer electrolyte membrane, whereas the produced electron (e−) is transferred to the cathode via an exterior circuit. When oxygen is fed to the cathode, the oxygen is bonded to the hydrogen ion (H+) and electron (e−), and is thus reduced, to produce water.
The polymer electrolyte membrane serves as a channel, transferring the hydrogen ion (H+) produced at the anode to the cathode and should be basically capable of conducing hydrogen ions (H+) well. In addition, the polymer electrolyte membrane should be efficiently capable of separating the hydrogen gas fed to the anode from the oxygen fed to the cathode, and requires excellent mechanical strength, dimensional stability and chemical resistance, as well as low ohmic loss at a high current density.
The currently used polymer electrolyte membrane includes a fluoride-based resin, more specifically, a perfluorosulfonic acid resin (hereinafter, referred to as “fluoride ion conductor”). However, the fluoride ion conductor has a problem in which pinholes are generated due to weak mechanical strength when used for a long time and energy conversion efficiency is thus deteriorated. In an attempt to reinforce mechanical strength, the thickness of fluoride ion conductor was increased. In this case, however, there are problems in which ohmic loss is increased, use of expensive materials is also increased and economic efficiency is thus deteriorated.
In order to solve these drawbacks of the fluoride ion conductor, recently, hydrocarbon-based ion conductors are actively developed. However, because the polymer electrolyte membrane repeatedly expands and contracts under humid/dry conditions which are operation conditions of the fuel cell, the hydrocarbon-based polymer electrolyte membrane structurally having a high moisture content has drawbacks of low long-term durability due to low dimensional stability and tensile strength.
To solve these drawbacks, a reinforcement membrane-type polymer electrolyte membrane which exhibits improved mechanical strength by introducing a support as a reinforcing material into the hydrocarbon-based ion conductor was suggested. The support is generally based on a hydrophobic hydrocarbon-based polymer having no ion conductivity. Due to the hydrophobic support, dimensional stability is enhanced, and as a result, mechanical properties such as tensile strength can be secured upon impregnation, and the membrane thickness to minimize membrane resistance and enhance functions can be minimized.
Meanwhile, in order to produce the hydrocarbon-based ion conductor in the form of a reinforcement membrane, the hydrocarbon-based ion conductor is dissolved in a solvent to prepare an impregnation solution, and the porous support is then immersed in the impregnation solution for a predetermined time, or the impregnation solution is applied to the surface of the porous support. However, in this case, due to low impregnability of the support, or deteriorated affinity between the hydrocarbon-based ion conductor and the porous support in the process of removing the solvent by evaporation after the impregnation or application, the porous support may have defects such as cavities. The corresponding membrane part is pressed due to the cavities and as a result, crack and membrane-electrode separation occur. For this reason, the impregnation or application should be inevitably repeated. This causes an increased and non-uniform thickness of the polymer electrolyte membrane.
In addition, when a support with a low porosity is used, cell performance is disadvantageously deteriorated because the support acts as resistance. In response to this problem, a reinforcement membrane having a porous support with maximized porosity was suggested. However, this reinforcement membrane exhibits poorer performance under low humidity conditions (less than 60%) than under high humidity conditions (60 to 100%), in spite of excellent performance and physical properties.